loader/dri3: Try to make sure we only process our own NotifyMSC events

We were using a sequence counter value to wait for a specific NotifyMSC
event. However, we can receive events from other clients as well, which
may already be using higher sequence numbers than us. In that case, we
could stop processing after an event from another client, which could
have been received significantly earlier. This would have multiple
undesirable effects:

* The computed MSC and UST values would be lower than they should be
* We could leave a growing number of NotifyMSC events from ourselves and
  other clients in XCB's special event queue

I ran into this with Firefox and Thunderbird, whose VSync threads both
seem to use the same window. The result was sluggish screen updates and
growing memory consumption in one of them.

Fix this by checking the XCB sequence number and MSC value of NotifyMSC
events, instead of using our own sequence number.

v2:
* Use the Present event ID for the sequence parameter of the
  PresentNotifyMSC request, as another safeguard against processing
  events from other clients
* Rebase on drawable mutex changes

Reviewed-by: Nicolai Hähnle <[email protected]> # v1

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